In OFDM (Orthogonal Frequency Division Multiplexing) wireless communication networks, data streams are typically transmitted in parallel using multiple orthogonal sub-carriers or tones within a single channel. The use of orthogonal sub-carriers allows the sub-carriers' spectra to overlap, thus achieving high spectrum efficiency. An OFDM system maps coded or modulated information symbols, QPSK (Quadrature Phase Shift Keying) or QAM (Quadrature Amplitude Modulation) symbols for instance, to sub-carriers in the frequency domain, and then generates a time domain signal for transmission using such a transformation technique as IFFT (Inverse Fast Fourier Transform). At a receiver, a time-to-frequency transformation, such as an FFT (Fast Fourier Transform), is used to convert a received time domain signal into the frequency domain. In order to recover transmitted source symbols correctly, the receiver aligns an FFT window with a corresponding IFFT window used at the transmitter and compensates for any frequency offset between the transmitter and the receiver.
Initial access to a communication network by a communication terminal involves a search operation to find available base stations and communication channels and a synchronization operation to synchronize the terminal to a base station. Dedicated physical channels, such as an initial access channel, a synchronization channel for timing and frequency synchronization, and a pilot channel to assist in channel estimation for coherent detection have been used for these operations, but increase communication signal overhead.
Other known network access techniques involve the insertion of a preamble or some other synchronization signal by a transmitter, at a predetermined location in a communication signal, and detection of the synchronization signal at a receiver. For synchronized communication networks, preambles from multiple base stations in the communication network are transmitted at the same time. Orthogonal preambles support efficient channel searching during initial access operations at a receiver. However, for asynchronous communication networks, synchronization signal transmissions from multiple base stations are not orthogonal, which degrades the performance of access algorithms based on synchronization signals. A further issue with preamble-based synchronization signal insertion schemes is that payload management may not be compatible with current standardized timing structures, such as the UTRAN (UMTS (Universal Mobile Telecommunication System) Terrestrial Radio Access Network) timing structure, i.e. the 3GPP (3rd Generation Partnership Project) TTI or transmit-time-interval. Specifically, the payload of a synchronization signal TTI would not be the same as the normal TTI.